This invention relates to an optic temperature sensor having a lens system for collimating a beam of light carried by a first array of bifurcated fiber bundle from a source to illuminating the entire surface of a fluorescent material to produce an output signal. The lens system focuses the output signal into a second array of the bifurcated fiber bundle to provide a detector with a signal corresponding to the temperature of the fluorescent material.
Fiber optic sensors have been used to measure temperatures in hostile environments. When a rare earth doped refractory oxide phosphor such as europium or fluorescent crystal such as chromium doped aluminum oxide is excited by a modulated light, it glows. The phenomenon of glowing or fluorescence arises when some of the energy of the photons absorbed and converted to vibrational energy while the remaining energy is emitted as photons of lower energy. An electron in the ground state is excited by the capture of an incident photon into a higher energy band. An incident photon may very quickly lose some energy through collisional nonradiative processes to occupy the lowest vibrational energy level in a higher energy band. If the quantum efficiency is relatively high very few excited electrons lose their energy through collisional processes and the electrons decay to the ground state with the simultaneous emission of a lower energy photon.
The lifetime of the excited electron varies from tens of nanoseconds to seconds. The luminescent decay time of the phosphor material is a function of the temperature of an environment. With known sensors, pulses of laser light are communicated through a fiber bundle to a remote tip located in a hostile environment. The tip contains a phosphor material, which is illuminated by the laser light. The luminescent decay time varies with the temperature of the tip. The glow or light emitted by the phosphor material travels back through the same fiber bundle to a coupler where it is diverted to a photodetector. The measurement of the decay time is an indication of the temperature of the tip.
Typically the fiber bundle is divided in the middle to establish first and second separate arrays. The first array carries the laser pulses to the phosphor material while the second array carries the light emitted by the phosphor material to the photodetector. This type fiber bundle is easy to manufacture and offers mechanical strength since the individual fibers are parallel to each other. However, it has been observed that most of the light emitted by the phosphor material occurs adjacent the first array with a proportional reduction of the light emitted by the phosphor material adjacent the second array through which an output is carried to the photodetector. In addition, a portion of the light emitted by phosphor material never enters the second array because the angle of entry is too oblique to enter the individual fibers.
In an effort to improve the communication of the output signal transmitted to the photodetector, it was decided to randomly distribute the first array and second array of fibers in the bundle. In this arrangement there is a greater possibility of having a receiver fiber near an exciter fiber and thus more of the fluorescent activity or output of the phosphor material could be communicated to the photodetector. While this arrangement does provide for a greater communication of the output signal from the phosphor material, the manufacture of such structure is more difficult. When the fibers cross over each other, internal void occurs and the outside diameter of the fiber bundle increases as compared with a bundle for the same number of fibers where the fibers are parallel to each other. In addition, during interweaving of the fibers, stress is placed on the fibers where they engage the other fibers and in some instance actual breakage in the individual fiber occurs. Once a break in a fiber occurs, the efficiency is proportionally reduced.